Techniques for accessing signals generated in circuits through test point taps are well known in the art. Such techniques may involve the collection of such test points at a connector for coupling to a remote location where they may be electrically contacted for analysis. Such contact normally does not cause electrical problems, although inadvertent short circuits may be introduced when an oscilloscope probe is applied to a test point. For a large scale computer installation in which signals are transmitted at high speed and the interference with any one signal during operation can cause havoc, access to lines and signals is very carefully controlled to avoid interference. As a result, direct access to signals on electrical coaxial cables between a CPU and a peripheral device is not available in an IBM computer environment.
In a large computer installation a large central processor unit (CPU) may be connected to external devices through a large number of coaxial cables. For example, in a large IBM main-frame computer installation a CPU having as many as 16 channels may have each of these coupled to a peripheral device such as a disk, printer, magnetic tape system or the like. With each channel there are two sets of coaxial cables to enable bidirectional data flow and are known as bus and tag cables. Each bus and tag set of cables may involve some twenty-one cables, though a higher number may be used. In many cases a number of such channels are coupled to a matrix switch where the channels may be connected to any one of a number of peripheral devices
Since, in a large CPU installation the signals on the cables are not normally accessible, access to these signals may be needed for trouble shooting or for evaluation and analysis.
In a typical IBM CPU main frame installation a number of peripheral devices are connected to CPU channels through bus and tag coaxial cables that are assembled in connectors and are connected into plugs at the peripheral devices as well as at the CPU channels. These cables are terminated at their ends with pluggable terminators that fit into plugs adjacent to and connected to plugs into which the bus and tag connectors are coupled. The terminators include a plurality of impedance matching elements that are respectively connected through the terminator and bus and tag plugs to respective coaxial bus and tag cables so as to terminate these with a minimum of electrical reflections. Typically, matching impedance elements in the form of resistors equal to the characteristic impedance of the coaxial cables are employed.
The terminators are fully enclosed devices, using passive circuits, and in an IBM main frame system do not enable direct access to the signals on the bus and tag cables. As a result technical problems, as these may arise and are believed related to the cabling system, are not easily resolved; particularly because the signal flow between an IBM CPU channel and an external device involves highly complex signals on many bus and tag lines. Hence, access to all bus and tag cables, about 42 lines, between a channel and an external device is likely to be needed for a complete signal analysis and an identification of a source of errors.
One technique for monitoring a channel of a data transmission circuit is described in U.S. Pat. No. 4,203,066 to Buck. This shows a monitor plug with a plurality of LED lamps and pin-jack access points within a plug for direct monitoring of signals in a patch module. The monitor module employs, as shown in FIGS. 9-12, a connector that plugs into an access opening, a printed circuit board and test jacks adjacent to LED's . The Buck device, however, is not suitable for providing direct access to the signals on bus and tag cables in an IBM mainframe system.
U.S. Pat. No. 3,659,273 to Knauft et al describes a device for checking the operation of peripheral devices. This device enables testing the operation by the addition of circuits that are active during particular times. U.S. Pat. No. 4,037,186 to Palmer et al describes a plug-in module or board to patch or switch electrical signals.
Neither Knauft nor Palmer describe a device that is suitable for providing a direct access to the signals at the ends of bus and tag coaxial cables at various places in an IBM main frame system.
Channel to peripheral matrix switches have been employed in an IBM computer mainframe system to enable a user to connect a channel to any one of a number of peripheral devices. Since access to the signals on the bus and tag coaxial cables is possible within the switch, bus or tag signal monitoring devices have been described for indicating, for example, channel activity. In such matrix switch the signals on the bus and tag cables are applied through so called receivers to the switching mechanism and outputted from the matrix switch through appropriate driver networks. The monitoring devices sample the signals on bus and tag lines as these are available between receivers and drivers. The monitoring devices include high input impedance isolation amplifiers whose inputs are connected to the bus and tag lines between the receivers and drivers.